Code division multiple access mobile communication system

ABSTRACT

In a mobile communication system using a code division multiple access (CDMA) method, spreading code detection and frame/slot timing synchronization (cell search) is conducted by using a long code masked symbol. The spreading factor of the long code masked symbol is set to a value lower than spreading factors of other ordinary symbols. As a result, it becomes possible to reduce the circuit scale and power dissipation of the mobile terminal and raise the speed of cell search.

This is a continuation application of U.S. patent application Ser. No.10/869,920 filed Jun. 18, 2004, now U.S. Pat. No. 7,778,310; which is acontinuation application of Ser. No. 09/518,690, filed Mar. 3, 2000, nowU.S. Pat. No. 6,879,571; which is a continuation application of U.S.Ser. No. 09/257,002, filed Feb. 25, 1999, now U.S. Pat. No. 6,507,576,the contents of which are hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a code division multiple access (CDMA)mobile communication system. In particular, the present inventionrelates to a cell search method using a long code masked symbol (searchcode) in perch channels.

2. Description of the Related Art

When a mobile terminal starts communication, or a mobile terminal movesfrom one base station area (cell) in which the mobile terminal iscurrently conducting communication to an adjacent cell (i.e., in thecase of hand over) in CDMA mobile communication systems, it is necessaryto conduct spread code detection or frame/slot timing synchronization.Such processing is called cell search.

As for an example of a conventional cell search method, a method ofspreading only one symbol located at the end of a slot by using aspecial short code called long code masked symbol instead of theordinary long code and short code is described in Technical Report ofIEICE (the Institute of Electronics, Information and CommunicationEngineers) DSP-96-116, SAT96-111, RCS96-122 (1997-01).

This cell search method using the long code masked symbol will now bedescribed. The cell search uses perch channels shown in FIG. 1. The term“perch channels” means control channels for notifying reverse linkinterference power measured at the base station, system frame number,and the like. Furthermore, the perch channels are transmitted alwayswith constant transmission power. Since a control signal of the perchchannels is used also as a reference signal of timing synchronizationconducted between the base station and the mobile terminal, the controlsignal of the perch channels is spread as described below. As for theperch channels, a first perch channel and a second perch channel aremultiplexed. In a long code masked symbol position (search codeposition) 101 of a first perch channel 106, a CSC (Common Short Code),i.e., a first search code 104 is mapped. In a long code masked symbolposition 101 of a second perch channel 107, a GISC (Group IdentificationShort Code), i.e., a second search code 105 is mapped. In a data symbolsection 102 (a section obtained by removing a long code masked symbolsection (search code section) from one slot section), a control signaltransmitted to the mobile terminal is spread by a long code and shortcode 103.

The long code is a long period spreading code assigned uniquely to thebase station. The short code is a short period spreading code assigneduniquely to each of channels under communication (including the controlchannel and transmission channel). The long code has a long code lengthand includes many kinds. In order to facilitate its timingsynchronization, therefore, the long code is classified into a pluralityof groups. The GISC is a short period code provided so as to correspondto the classification of the long code. In the case where the mobileterminal is to conduct timing synchronization of the perch channels, themobile terminal lightens the load of synchronization of the long codeused by the base station (i.e., decreases time, circuit means, electricpower, etc. required for the timing synchronization), by detecting theGISC and narrowing down the long code to a fixed range (i.e., bylimiting candidates for the long code which may be used). The CSC is ashort period spreading code defined uniquely to the mobile communicationsystem.

The detection of the long code and the frame/slot timing used by thebase station, utilizing the perch channels is conducted as follows: (1)the mobile terminal despreads the perch channels by using the CSC, anddetects the slot timing on the basis of the height of the correlationvalue; (2) the mobile terminal conducts despreading in all GISCs inconformity to the synchronized slot timing, and detects the GISC on thebasis of the height of the correlation value; (3) the mobile terminalconducts despreading by using all long codes belonging to a groupassociated with the GISC, and detects the long code on the basis of theheight of the correlation value.

The format and transmission power of the perch channels of theconventional method are shown in FIG. 2. The symbol rate of the perchchannels is 16 kbps (spreading factor being 256) and constant in allsections including the long code masked symbol. In the long code maskedsymbol section in which the second perch channel is transmitted,transmission power P1 of the first perch channel is lowered bytransmission power P2 of the second perch channel. Thereby, transmissionpower of the perch channels after multiplexing is constant.

In the conventional system which conducts spreading process in the longcode masked symbol section at the same symbol rate as in the data symbolsection, it took the longest time in a first stage (slot timingsynchronization) of the cell search. In order to conduct timingsynchronization in a short time, a matched filter (MF) capable ofderiving correlation results at a plurality of timing instants at onceis used in many cases.

FIG. 13 shows time required in each stage of the cell search in the casewhere cell search is conducted by despreading the perch channels havinga spreading factor of 256, by use of a MF with 64 chips. The stagerequiring the longest time is slot timing synchronization 1301. Forattaining faster cell search, it is an indispensable subject to shortenthe time required for timing synchronization. In timing synchronizationusing the MF, correlation values at all timing instants in one symbol(256 chips) section are accumulated by using CSCs of a plurality ofslots, thereby conducting slot timing synchronization at high precision.For example, correlation values derived for CSCs of 48 slots areaccumulated. In FIG. 13, one accumulation value with respect to timinginstants of 64 chips which is the same in number as the number of tapsof the MF is derived in one cycle 1301 of timing synchronization.

If the MF with 64 taps is used, coefficient mode switchover becomesnecessary in order to derive correlation values at all timing instants.This results in a problem that the time required for timingsynchronization, in turn the time required for cell search becomeslonger. On the other hand, if a MF with 256 taps is used, then thereceived signal can be despread with coefficients corresponding to onesymbol set in the MF intact. Since the coefficient mode switchover thusbecomes unnecessary, correlation at all timing instants can be derivedat high speed. However, both the gate size and power consumption of theMF become very large.

SUMMARY OF THE INVENTION

In order to conduct the cell search at high speed while suppressing thegate size and the power consumption, the spreading factor of the longcode masked symbol is made smaller than spreading factors of otherportions of the perch channels.

In particular, a symbol rate according to typical number of taps of theMF used in the mobile terminal is determined. For example, in the casewhere the spreading factor of the mask symbol is 64, timingsynchronization is conducted by using a MF with 64 taps. In this case,the symbol length coincides with the number of taps of the MF. Withcoefficients corresponding to one symbol set in the MF intact,therefore, it is possible to conduct despreading of the received signaland conduct search of all timing instants in the 64 chip section.Without increasing the gate size and power consumption, fast cell searchthus becomes possible.

By referring to detailed description of preferred embodiments describedbelow and accompanied drawing, these or other objects, features, andadvantages will become more apparent.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing a channel format of perch channels;

FIG. 2 is a diagram showing a channel format and transmission power ofperch channels of a conventional system;

FIG. 3 is a diagram showing a channel format and transmission power ofperch channels of a first embodiment;

FIG. 4 is a diagram showing a channel format and transmission power ofperch channels of a second embodiment;

FIG. 5 is a diagram showing a channel format and transmission power ofperch channels of a third embodiment;

FIG. 6 is a diagram showing a channel format and transmission power ofperch channels of a fourth embodiment;

FIG. 7 is a diagram showing shortening of the search time, and reductionof the circuit scale and transmission power;

FIG. 8 is a configuration diagram of a mobile terminal;

FIG. 9 is a diagram showing a configuration example of a cell searchtiming synchronization unit of a mobile terminal;

FIG. 10 is a diagram showing a configuration example of a cell searchGISC detection unit of a mobile terminal;

FIG. 11 is a diagram showing a configuration example of a first longcode detection unit of a mobile terminal;

FIG. 12 is a diagram showing a configuration example of a second longcode detection unit of a mobile terminal; and

FIG. 13 is a diagram showing time required at each stage of the cellsearch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, the configuration of a mobile terminal used in a CDMAmobile communication system according to the present invention will bedescribed by referring to FIG. 8. A received signal of a carrierfrequency received from an antenna is lowered in frequency by an RF unit801. The received signal of the baseband is inputted to a cell searcher805 and a receiver 804 via an RF interface 802. The cell searcher 805conducts the above described cell search. The receiver 804 conductsdespreading, error correction and the like of physical channels otherthan the perch channels. The decoded received signal is outputted via auser interface 807, and subjected to subsequent processing. Atransmission signal to be transmitted to the base station is inputted toa transmitter 803 via the user interface 807. The transmitter 803conducts coding and spreading of the transmission signal. A controller806 conducts initial value setting in various units and timingmanagement by using a DSP (Digital Signal Processor).

FIGS. 9 to 12 show configuration examples of blocks 810-812 of FIG. 8.FIG. 9 shows the configuration of a timing synchronizer 810. In thetiming synchronizer 810, it is necessary to derive correlation values oftiming corresponding to one symbol. Therefore, an MF 901 capable ofproviding correlation results at a plurality of timing instants at atime is used. As for coefficients of the MF 901, CSC generated from aCSC encoder 902 is used. An accumulator 903 accumulates correlationvalues outputted from the MF for a plurality of slots. A peak detector904 detects such a timing as to maximize the accumulated correlationvalues, as slot timing.

FIG. 10 shows a configuration example of a GISC detection unit 811. FIG.11 shows a configuration example of a first long code detection unit.FIG. 12 shows a configuration example of a second long code detectionunit. A long code detection unit 812 includes a first long codedetection unit and a second long code detection unit. In these circuits,frame/slot timing is already known by a timing detection unit. Byarranging correlators 1001 in parallel for conducting despreading at onedetected timing instant, high speed processing can be conductedefficiently.

The GISC detection unit 811 (FIG. 10) stores a received signal of a longcode masked symbol in a RAM 1002. GISCs are specified in a GISC encoder1003 one after another by the DSP. Correlation for each chip is thusderived. A correlation value in one symbol is derived by an accumulator1004. Such processing can be conducted at high speed by suitablyconducting parallel processing. By selecting the highest one of thederived correlation values, the GISC is detected.

The first long code detection unit (FIG. 11) calculates correlationvalues over approximately 10 symbols, and detects a long code used bythe base station out of long codes belonging to a class corresponding tothe detected GISC. Long codes specified in a long code generator 1102one after another by the DSP are multiplied by a short code of the perchchannels generated by a short code generator 1103. Correlation of eachtiming is derived by a correlator 1001. Correlation values correspondingto 10 symbols are accumulated by an accumulator 1101. This processing isconducted in parallel with different long codes. On the basis of aresult of accumulation of correlation values over approximately 10symbols, a probable long code is designated.

For the long code designated by the first long code detection unit, thesecond long code detection unit (FIG. 12) conducts processing similar tothat of the first long code detection unit over one frame section andoutputs the result to delay locked loop 813. In the case where apredetermined accumulation value has been obtained, the cell search iscompleted.

A CDMA communication system performing a cell search method using thelong code mask symbol will now be described centering around an examplein which only the long code masked symbol portion of the perch channelstypically transmitted at 16 Ksps (spreading factor 256) is made to havea spreading factor of 64.

The spreading factor is not limited to 64. Similar effects can beobtained so long as the spreading factor is less than 256.

As a first embodiment, FIG. 3 shows a channel format and transmissionpower in the case where spreading factors of the CSC and GISC are madesmaller (64 in the example) than those of other symbols of the perchchannels, and the CSC and GISC are inserted at different timinginstants. In order to prevent other ordinary symbol portions from beingaffected, a masked symbol section 131 is made to have 256 chips in thesame way as the conventional system. The CSC and GISC may be inserted inany section of four sections (133, 134, 135 and 136) obtained bydividing the mask symbol section at intervals of 64 chips. In the casewhere the symbol length of the GISC becomes short and consequently thenumber of GISCs is not enough for the number of classes of the long codewhich GISCs are assigned to, it is also possible to adopt such a methodthat long code identification groups are sorted out according to whichof the four insertion sections they are inserted. In the masked symbolsection, sections other than those of CSC and GISC are provided with nosymbols.

If the symbol length is shortened, the number of times of possibleaccumulation times decreases. For obtaining the same receivingsensitivity, therefore, the transmission power must be raised. However,the perch channels are always subjected to transmission with constantpower. In addition, the long code masked symbol portion is poor inorthogonality, and therefore, tends to exert interference power to otherchannels. Therefore, it is desirable to suppress the transmission poweras low as possible. In the present embodiment, therefore, the CSC andGISC are not multiplexed, but the CSC and GISC are transmitted by timedivision in the long code masked symbol portion. Even if the spreadingfactor is reduced to ¼ at this time, transmission power P3 of the CSC istwice the transmission power P1 of the conventional technique and thesame reception sensitivity is obtained. The same is true of thetransmission power P4 of the GISC.

As a second embodiment, FIG. 4 shows a channel format and transmissionpower in the case where the spreading factors of the CSC and GISC aremade sufficiently small (16 in the example) as compared with othersymbols of the perch channels, and the CSC and GISC are multiplexed andtransmitted. It is necessary to make transmission power P5 of the CSCand transmission power P6 of the GISC large so as to correspond to thespreading factors. If the symbol rate of channels other than perchchannels is fast, then the number of perch channels which are affectedby the fact that the perch channel power is increased will become large.In such a case, by multiplexing the CSC and GISC to shorten the sectionin which the transmission power becomes large as in the presentembodiment, although the influence of the perch channels on otherchannels may be large, the shortening of the affecting symbol sectionsurely causes influence as a whole to be lightened.

As a third embodiment, FIG. 5 shows a channel format and transmissionpower in the case where the spreading factors of the CSC and GISC aremade sufficiently small (64 in the example) as compared with othersymbols of the perch channels, and the GISC is repeated a plurality oftimes (three time in the example). By transmitting the GISC repetitivelyn times, the number of accumulation times is increased, and accordinglytransmission power P8 of the GISC of one time is equal to 1/n oftransmission power P7 of the CSC. As a result, influence on otherchannels is suppressed.

As a fourth embodiment, FIG. 6 shows a channel format and transmissionpower in the case where the spreading factor of the CSC is made smallerthan that of the GISC (in the example, the spreading factor of the CSCis 64 and the spreading factor of the GISC is 256). In the abovedescribed three stages of the cell search, the GISC detection can beconducted by despreading only at timing designated from the CSC, and acorrelator is used instead of the MF in many cases (as shown in FIG. 10,for example). As in the present embodiment, therefore, the speed of thesearch can be raised while suppressing the interference on otherchannels, by making the spreading factor of the CSC affecting the numberof taps of the MF small and making the spreading factor of the GISClarger than it in order to suppress the transmission power.

In FIG. 7, there is shown a list of time required at each stage of thecell search obtained when the spreading factor of the long code maskedsymbol and the number of taps of the MF are changed.

By thus making the spreading factor of the long code masked symbolsmall, the time required for timing synchronization can be made shorterthan that of the conventional method, and the number of taps of the MFcan be shortened, resulting in reduced gate size and power consumption.

The present invention has been disclosed in connection with thepreferred embodiments. Those skilled in the art can apply variousmodifications to the embodiments on the basis of the disclosure. Allmodifications existing within the true spirit and scope of the presentinvention are incorporated in the claims.

1. A mobile terminal used in a code division multiple access mobilecommunication system, in which a base station transmits a control signalvia a perch channel formed such that a long period code assigned to saidbase station and a first short period code are mapped in a first sectionof one slot of said perch channel and a predetermined short period codeis mapped in a second section of said one slot, said mobile terminalcomprising: a radio frequency (RF) unit for converting a received signalof a carrier frequency received from an antenna to a baseband signal;and a correlator connected to said RF unit and arranged to receive saidbaseband signal, the correlator including a code generator forgenerating said predetermined short period code and arranged tocalculate a correlation value for said baseband signal by using saidpredetermined short period code to establish slot timingsynchronization, wherein a symbol length of said predetermined shortperiod code has a smaller value than a symbol length of said first shortperiod, and wherein in said second section are mapped a second shortperiod code, and a third short period code being one of a plurality ofshort period codes each corresponding to classification of the longperiod code spreading said first section.
 2. A mobile terminal accordingto claim 1, wherein said second short period code is a short period codecommon to base stations included in the mobile communication system, andsaid third short period code is one of a plurality of short periodspreading codes each corresponding to classification of said long periodcode.
 3. A mobile terminal used in a code division multiple accessmobile communication system in which a base station transmits a controlsignal via a per channel formed such that a long period code assigned tosaid base station and a first short period code is mapped in a firstsection of one slot of said perch channel and a second short period codeand a third short period code are mapped in a second section of said oneslot, comprising: a radio frequency (RF) unit for converting a receivedsignal of a carrier frequency received from an antenna to a basebandsignal; and a correlator including a code generator arranged to generatesaid second short period code in response to a timing signal forcalculating a correlation value for said baseband signal, wherein saidreceived signal includes said control signal, said long period codebeing assigned to said base station and said first short period codebeing assigned to each channel of said base station, and said secondshort period code having a spreading factor smaller than said firstshort period code and said third short period code having a spreadingfactor not greater than said first short period code, and wherein saidcorrelator calculates the correlation value for said control signal byuse of said second short period code.
 4. A mobile terminal used in acode division multiple access mobile communication system, comprising:radio frequency (RF) unit for converting a received signal of a carrierfrequency received from an antenna to a baseband signal; and acorrelator including a code generator arranged to generate apredetermined short period code in response to a control signal, thecorrelator for calculating a correlation value for said received signalusing said predetermined short period code, wherein said received signalincludes said control signal, a first section of one slot of saidcontrol signal having mapped in it a long period code assigned to saidbase station and a short period code assigned to each channel of saidbase station, a second section of said one slot having mapped in it saidpredetermined short period spreading code, and a period of saidpredetermined short period code is smaller than a period of said shortperiod code mapped in said first section.